Conventional raise boring begins with the drilling of a pilot hole vertically down, typically using a directional drilling system. It is drilled using a drilling unit at the surface from which a hollow drill string, comprising a plurality of drill pipes fitted together, extends downwardly. A roller bit to drill the pilot hole is fitted to the lowermost drill pipe of the drill string, with the pipes having a standard thread for high-torque applications. After the pilot hole has broken through to a lower level, the roller bit is removed and replaced with a reamer head comprising a plurality of cutters. The reamer head is rotated and pulled back towards the surface-mounted drilling unit so as to cut a larger hole, or raise, through the ground and rock. The cuttings fall by gravity into a chamber at the bottom of the hole, typically in an uncontrolled manner, where they are removed using a loader.
Blind hole boring, on the other hand, comprises drilling an oversized pilot hole. The oversized pilot hole can be drilled either in a single step, or, more typically, by first drilling an initial 400 mm pilot hole, for example, which is then subsequently enlarged to define a 3 m oversized pilot hole. This process is reasonably well known in the art. A cutting head is then installed above the drilled oversized pilot hole, so that drilling can occur downwardly. The cuttings are then flushed out of the oversized pilot hole. This particular technique is not used that often, as the risk of blocking the pilot hole and creating mud rushes at the bottom of the hole is relatively high.
No known boring system is capable of boring relatively larger holes (preferably having a diameter of between 8 and 15 metres, but possibly even larger), with the cuttings being removable from above the boring system without having to flush out the cuttings, using, for example, reverse circulation.
There are a number of related prior art documents, including published PCT patent application no. WO9320325 which discloses a down reaming apparatus having an upper stabilizer which supports the down reaming apparatus in a bored hole, and a lower stabilizer that provides additional support for the down reaming apparatus.
U.S. Pat. No. 3,965,995 discloses a machine for boring a large diameter blind hole, the machine including a cutterwheel mounted at the lower end of the machine for rotation about a horizontal tubular support. A gripper assembly, positioned above the cutterwheel, secures the machine against the tunnel wall. U.S. Pat. No. 4,646,853 discloses a substantially similar machine.
The prior art documents listed and described above are just a selection of known documents that disclose, to varying degrees, the broad concept of down reaming. However, they all tend to suffer from the following disadvantages:                1. None of them disclose easily implementable arrangements for removing the reamed cuttings from above the reaming apparatus i.e. so that the cuttings exit out of the top of the bored hole.        2. None of them disclose a gripping arrangement that would enable the relatively larger holes (with diameters of between 8 and 15 metres) to be safely, efficiently and economically drilled.        3. None of them disclose a scalable apparatus, to enable a single reaming or boring apparatus to be modified to bore shafts of varying diameters.        4. None of them disclose the ability to bore through hard rock, which presents particular difficulties. At the moment, hard rock boring involves using a blast and advance sequence, which the present invention specifically aims to avoid.        
It is an aim of the present invention to provide a boring system or rig to address the above shortcomings prevalent in existing boring arrangements. In an embodiment, the aim is to provide a blind shaft boring system that can achieve very accurate directional drilling and avoid having to drill an initial pilot hole, as is conventionally done.